Radiological interpretation

Elizabeth Stokell MA VetMB CertVR MRCVS
Department of Clinical Veterinary Medicine
University of Cambridge
Madingley Road
Cambridge

1. Introduction

1. X-Ray photons have the potential to penetrate tissue
2. X-Ray photons will be attenuated in part by the tissue, and in part will pass through the tissue to interact with and expose the radiographic film
3. Absorption of X-Rays is a function of the atomic number and thickness of the tissues/objects
• Tissues/objects with a higher atomic number will absorb more radiation than tissues with a lower atomic number
• Thicker tissue/objects will absorb more X-Rays than thinner tissue of similar composition
4. The greater the amount of tissue absorption, the fewer X-Ray photons reach the film, and the whiter the image on the film
5. The radiograph will display a range of densities from white, through various shades of grey, to black
• Radiopaque tissues/objects appear more white
• Radiolucent tissues/objects appear more black
6. The resultant pattern of opacities forms an image on the radiograph, which is recognisable in form, and which can be interpreted

2. Radiopacity

1. Atomic number
• The higher the atomic number, the more radiopaque the tissue/object:
2. Physical opacity
• Air, fluid and soft tissue have approximately the same atomic number, but the specific gravity of air is only 0.001, whereas that of fluid and soft tissue is 1
• Therefore air will appear black on a radiograph, compared with fluid and soft tissue, which appear more grey
3. Thickness
• The thicker the tissue/object, the greater the attenuation of X-Rays and the more white the image will be
• When two tissues/objects are superimposed, the composite shadow formed by these will appear more opaque than either of the two separate tissues/objects (e.g. the area where the two kidneys overlap appears more radiopaque than either kidney itself)

3. Basic tissue radiographic opacities

1. Mineral opacity
• Bone is composed primarily of calcium and phosphorus
• There is a normal variation in radiopacity within the same bone and between bones because of the difference in radiopacity of:
• Compact vs spongy bone
• Trabecular bone vs intertrabecular spaces
• Cortical bone vs medullary canal
• Diseased bone may be more (sclerotic) or less (porotic) opaque than normal bone
2. Soft tissue/fluid opacity
• Both soft tissues and fluids have the same radiopacity
• This is the radiopacity of normal soft tissue and fluid-filled organs (heart, liver, spleen, urinary bladder)
• Variation in volume, thickness and degree of compactness of soft tissue creates a pattern of various densities on the radiograph
3. Fat opacity
• Fat is more lucent than bone or soft tissue, but is more opaque than gas
• Fat produces radiographic contrast for differentiation and visualisation of many organs and structures, in that fat surrounding an organ or structure will allow it to be delineated
• In immature and thin animals, the lack of fat results in poorer contrast in the radiograph
4. Gas opacity
• Gas is the most radiolucent material visible on a film
• This lucency provides contrast to allow visualisation of various structures, e.g. the heart and great vessels outlined against the air-filled lungs in the chest.
5. Metal opacity
• This is the most opaque shadow seen on radiographs, and may be seen as:
• Contrast media: barium, water-soluble iodine media
• Orthopaedic implants
• Metallic foreign bodies
• Artefacts, e.g. metal on collar and lead chain
6. Only these five radiographic opacities are visible on a radiograph
• However, there is some variation in opacity within each group
• The appearance of these opacities is relative
• For instance, a small cystic calculus (mineral opacity) may be difficult to identify in a bladder full of urine (soft tissue opacity), but will be more readily apparent in a pneumocystogram since it contrasts with the air (gas opacity) in the bladder
• In a positive contrast cystogram the calculus will appear relatively radiolucent as it is less opaque than the iodine-containing contrast medium (metal opacity)

4. Radiologic interpretation

1. Viewing the radiograph
• This should be done in a quiet, darkened room
• At least two good, evenly-lit viewing boxes are required
• A bright light illuminator is required for relatively over-exposed areas
• The bright edges around the film should be masked
2. Three-dimensional concept
• The radiographic image is simply a two-dimensional shadowgram of the patient
• The third dimension must be reconstructed mentally, preferably from two radiographic projections made at right angles (orthogonal projections) to each other
• Oblique projections may be required to assess anatomically complicated areas
• Reference to normal should be made through the use of anatomic specimens, radiographs of normal animals and textbooks.
3. Routine assessment of radiographs
• Ensure that the radiograph is the one of the patient being examined, and check the date
• Ensure two orthogonal projections are available
• The radiographic views are named according to the direction the primary beam enters and leaves the tissue and the body part being examined (e.g. mediolateral view of stifle joint, dorsopalmar view of carpus)
• The position of the patient during exposure should be known, and left/right markers should be identified
• The radiograph should be of high technical quality with respect to positioning, centring, collimation, exposure and development, and should be free from artefacts
4. Every shadow visible must be evaluated to determine whether it is:
• A feature of normal anatomy
• A composite structure formed by superimposition of structures
• An artefact produced by inaccurate positioning
• A pathologic lesion: a)-c) must be ruled out first

5. Evaluating the radiographs

1. Determine whether an abnormality exists:
• This is often the most difficult part
• There is a wide range of normal anatomic variants
• It is impossible to remember, or even see, in a lifetime all the normal variations
• Reference should be made to textbooks, normal radiographs, tissue specimens or the contralateral limb
• This decision making improves with experience
2. Define the anatomic location of the abnormality
• A minimum of two orthogonal projections are required
• More views may be required in certain areas
3. Classify the abnormality according to its roentgen signs
• The roentgen signs are defined below
4. Make a list of differential diagnoses (gamuts) by considering what diseases could cause the observed roentgen signs
• For example, if the roentgen sign is the presence of an enlarged kidney, then possible differentials (gamuts) are:
• neoplasia
• hydronephrosis
• cyst
• abscess
• subcapsular urine/haemorrhage
5. If a number of abnormal roentgen signs are identified, then those gamuts common to all lists are more likely (assuming only one problem is present)

6. Description of radiologic abnormalities of tissues/organs/objects (roentgen signs)

1. Changes in size of an organ or structure
• Increase in size e.g. diffuse neoplasia within the spleen
• Decrease in size e.g. poor development of the liver with a portosystemic shunt
2. Variation in contour or shape
• This may be local or diffuse e.g. hypertrophy of chambers of the heart in cardiomyopathy
• Irregularity of mucosal border seen on positive contrast study with intestinal lymphosarcoma
3. Variation in number of organs
• Many organs or structures may be present in increased or decreased number or absent completely (e.g. supernumerary teeth and ribs, absence of a kidney)
4. Change in position of an organ or structure
• Presence of abdominal organs in chest in diaphragmatic rupture
• Ventral deviation of descending colon by enlarged sub-lumbar lymph nodes
5. Alteration in opacity of an organ or structure
• Increased radiopacity
• increased opacity in air-filled space e.g. fluid-filled tympanic bulla
• calcification within soft tissues
• radiopaque foreign body
• Increased lucency
• gas in abnormal sites e.g. subcutaneous emphysema
• bone may appear more lucent with osteoporosis, osteomyelitis and neoplasia
6. Alteration in the architectural pattern of an organ or structure
• Change in normal bone trabeculation, or bronchovascular markings in the lungs
7. Alteration in the normal function of an organ
• Secretory contrast studies e.g. excretory intravenous urogram
• Transit contrast studies e.g. Barium series
• Physiologic phases e.g. inspiratory and expiratory chest films
• Moving picture image intensification contrast studies of pharynx and oesophagus

7. Other clues

1. Summation shadows
• This results when parts of a patient or an object in different planes are superimposed
• The result is a summation image representing the degree of X-Ray absorption by all the superimposed objects
• Radiolucent summation shadows are formed in the 'Swiss cheese ' effect
• When a radiograph is made of, for example, a block of Swiss cheese, fewer X-rays are absorbed by the cheese in areas where the cavities overlap.
• The more cavities that overlap, the greater the number of X-Rays that reach the film.
• Radiopaque summation shadows are involved in the 'bunch of grapes' effect
• A radiograph of a single small object, for instance a grape, may not be readily visible
• If a radiograph is made of a bunch of grapes, the areas where many grapes overlap will absorb more X-Rays
• This feature accounts for the visibility of miliary pulmonary metastases, where the individual size of the metastases is very small
2. The silhouette effect
• This principle is based on the fact that when two structures of the same radiopacity
are in contact, their individual margins at the point of contact cannot be distinguished.
• For instance, the liver and stomach are generally in close contact and a composite shadow representing both structures is formed on a radiograph
• A coronary artery and a small pulmonary artery of the same size are not equally visible on a thoracic radiograph
• The coronary artery is not visible since it has the same radiopacity as the heart, and there is no intervening tissue of a different radiopacity
• The pulmonary artery is visible because it is not in contact with the heart, and it is surrounded by the more radiolucent lung
• Conversely, if two objects are not in contact, and are separated by a substance of different radiopacity, their borders can be distinguished
• If the two objects are separated along the axis of the primary beam, then a summation shadow will be formed
• For example, overlap of the renal shadows is often identified on a lateral abdominal radiograph
• If the two objects are separated along a plane perpendicular to the axis of the primary beam then an obvious space is seen between them
• For example, a lucent space is identified between the borders of the heart and diaphragm on an inspiratory thoracic radiograph
• If two structures of the same radiopacity are in contact, one is said to silhouette with the other, or to form a positive silhouette sign. This terminology is confusing, and the term 'border effacement' has been suggested when their is a loss of the clear margins of a structure
3. Importance of a contrasting substance
• Just as the lack of a contrasting material prevents distinguishing between two structures of the same radiopacity, the presence of a contrasting substance allows some structures to become exquisitely visible
• This principle is particularly important when the contrasting substance is air, and the object in question is on the surface of the body
• For example, in many patients, nipples and the prepuce are clearly visible in ventrodorsal projections of the abdomen
• These structures are not particularly large or radiopaque, but cast a disproportionately opaque shadow
• The explanation lies in the fact that these structures are surrounded by air, and their margins are parallel to the axis of the central ray, providing optimum geometry for visualisation
4. Perception
• When evaluating radiographs, the eyes are used to detect abnormalities which are interpreted by the brain
• However, the eyes and brain do not always perceive appearances accurately, and optical illusions may occur
• What appears as concrete visual evidence is not always such, and perception is an important part of radiographic interpretation
• What appears to be an obvious finding to inexperienced radiologists may be an incorrect assessment because of perception

8. Pitfalls in interpretation

1. The presence of an obvious abnormality that distracts the evaluator from systematic evaluation of the rest of the radiograph
2. Discovery of a lesion that answers the clinical question that prompted the radiographic examination, thereby distracting the evaluator
3. Tunnel vision, which is a preconception of what will be found, so that when the preconception is confirmed, viewing of the radiograph ends
4. Failure to adopt a systematic approach, and using the error-prone 'Aunt Minny' approach;
• Aunt Minny represents an abnormality which looks like one that the evaluator has seen before, or been told about (e.g. distal radial metaphyseal osteosarcoma)
• Uncle Fred represents a boring abnormality which is often present (e.g. ventral lumbar spondylosis)
• Cousin Harry represents an abnormality which the evaluator has not seen for a long time, but would like to see (e.g. peritoneopericardiac diaphragmatic hernia)
5. The 'Aunt Minny' approach has its devotees, and those who use it often appear to have a supernatural ability to make a diagnosis
• However, there is more than one possible cause for most abnormalities seen
• It would be impossible to remember the appearance of every disease
• It would be difficult to recognise new findings using this approach